In these years, it has been desired to save energy in terms of environmental measures as well. In apparatuses using batteries, such as cellular phones and digital cameras, it has become increasingly important to reduce power consumed inside the apparatus in terms of a longer battery service life as well. Therefore, non-isolated DC-DC converters using an inductor, which are highly efficient and also reducible in size, are often used as power supply circuits.
Further, apparatuses have become more sophisticated, so that, for example, video has been recorded and reproduced more frequently than still images. As a result, CPUs used in apparatuses have become higher in performance to operate at higher clock frequencies. However, higher clock frequencies cause the problem of an increase in current consumption proportional to the increase in clock frequency. Further, higher clock frequencies require higher supply voltage. This results in a substantial increase in power consumption. Therefore, supply voltage is reduced for lower clock frequencies at the time of regular operations, and supply voltage is increased for higher clock frequencies only when high-speed processing is necessary as in the case of processing video, thereby minimizing an increase in power consumption.
In order to meet such requirements, DC-DC converters capable of changing output voltage in accordance with a voltage setting signal are needed.
As a method for changing output voltage, the method illustrated in Japanese Laid-Open Patent Application No. 2007-116804 is known, where the output voltage of a d/a converter is used as a reference voltage used for a DC-DC converter and a voltage setting signal fed from a control circuit such as a CPU is input to the d/a converter, thereby changing the output voltage of the d/a converter.
However, if the same clock signal is used for causing the d/a converter to operate and for switching the DC-DC converter, this results in the problem of reduction in response speed in changing the output voltage of the DC-DC converter.
A description is given in more detail of this problem. FIG. 1 is a circuit diagram of a conventional DC-DC converter using the output voltage of a d/a converter as a reference voltage.
Referring to FIG. 1, a DC-DC converter 100 is, which is a current-mode controlled DC-DC converter, includes an oscillator circuit (OSC) 111, a d/a converter (DAC) 112, an error amplifier circuit (AMP) 113, a pulse-width modulation (PWM) comparator (CMP) 114, an inductor current detector circuit (I/V) 115, an R-S flip-flop circuit (RSFF) 116, a drive control circuit 117, a switching transistor M1, a synchronous rectification transistor M2, an inductor L11, a capacitor C11, a resistor R11, and a resistor R12.
The oscillator circuit 111 outputs a clock signal CLK, which is input to the d/a converter 112 and the SET input S of the R-S flip-flop circuit 116.
FIG. 2 is a diagram illustrating the d/a converter 112. A description is given, with reference to FIG. 2, of the d/a converter 112.
Referring to FIG. 2, the d/a converter 112 includes an up/down counter 121, a digital comparator 122, a set voltage register 123, a code-voltage converter circuit 124, and an AND circuit 125.
The up/down counter 121 counts pulses of the clock signal CLK input through the AND circuit 125, and outputs the result of the counting to the digital comparator 122 and the code-voltage converter circuit 124 as an output code signal. Further, the up/down counter 121 counts up or counts down depending on an up/down signal U/D.
The set voltage register 123 stores a voltage setting signal Voset output by a control circuit (not graphically illustrated), and outputs the value of the voltage setting signal Voset to the digital comparator 122 as an output code signal.
The digital comparator 122 compares the output code of the set voltage register 123 and the output code of the up/down counter 121. The level of the signal output from the output terminal DCout of the digital comparator 122 is LOW if the output codes are equal or HIGH if the output codes are different. Further, the up/down signal U/D is switched to DOWN if the output code of the up/down counter 121 is greater than the output code of the set voltage register 123, and to UP if the output code of the up/down counter 121 is smaller than the output code of the set voltage register 123.
The code-voltage converter circuit 124 converts the output code of the up/down counter 121 into voltage, and outputs the voltage. This output is the output of the d/a converter 112 and is used as the reference voltage Vref of the DC-DC converter 100.
The output of the d/a converter 112 is connected to the non-inverting input of the error amplifier circuit 113. A voltage Vfb generated by dividing the output voltage Vo of the DC-DC converter 100 between the resistor R11 and the resistor R12 is input to the inverting input of the error amplifier circuit 113. The output of the error amplifier circuit 113 is connected to the inverting input of the PWM comparator 114. The output of the inductor current detector circuit 115 is connected to the non-inverting input of the PWM comparator 114.
The inductor current detector circuit 115 converts a current that flows through the inductor L11 into voltage, and outputs a ramp voltage subjected to slope compensation for preventing subharmonic oscillation.
The output level of the PWM comparator 114 becomes HIGH to reset the R-S flip-flop circuit 116 when the output of the inductor current detector circuit 115 reaches the output voltage of the error amplifier circuit 113.
The R-S flip-flop circuit 116 is set to turn its Q output HIGH when the level of the clock signal CLK input to its SET input S becomes HIGH. The Q output is connected to the drive control circuit 117.
When the input level of the drive control circuit 117 becomes HIGH, the drive control circuit 117 turns a control signal PHS LOW. On the other hand, when the input level of the drive control circuit 117 becomes LOW, the drive control circuit 117 turns the control signal PHS HIGH.
The control signal PHS is input to the gate of the switching transistor M1, which is a p-channel MOS transistor (hereinafter, PMOSFET), so that the switching transistor M1 turns ON or OFF in accordance with the control signal PHS.
The synchronous rectification transistor M2, which is an n-channel MOS transistor (hereinafter, NMOSFET), is controlled so as to turn ON or OFF complementarily to the switching transistor M1.
It is assumed that the voltage setting signal Voset is output by a control circuit (not graphically illustrated) so that a new target value for the reference voltage Vref is written to the set voltage register 123. In this case, the level at the output terminal DCout of the digital comparator 122 becomes HIGH so as to open the gate of the AND circuit 125. As a result, the clock signal CLK is applied to the clock input of the up/down counter 121. If the new target voltage is higher than the previous one, the digital comparator 122 outputs an UP signal. As a result, the up/down counter 121 counts up.
The code-voltage converter circuit 124 outputs a voltage according to the output code of the up/down counter 121 as the output voltage (reference voltage Vref) of the d/a converter 112. Therefore, every time a pulse of the clock signal CLK is input to the up/down counter 121, the output voltage (Vref) of the d/a converter 112 is changed for one bit.
FIG. 3 is a timing chart of signals illustrating the clock signal CLK and the operations of the d/a converter 112 and the DC-DC converter 100. For explanatory purposes, pulses of the clock signal CLK are numbered P1 through P6.
In the case of increasing the reference voltage Vref, the voltage setting signal Voset causes the code of a voltage higher than the current value of the reference voltage Vref to be written to the set voltage register 123. Then, the level at the output terminal DCout of the digital comparator 122 becomes HIGH so as to open the gate of the AND circuit 125. As a result, the clock signal CLK is input to the up/down counter 121. Further, since the digital comparator 122 outputs an UP signal, the up/down counter 121 counts up.
When the clock signal CLK becomes HIGH (in level) at P1, the count of the up/down counter 121 is incremented by one, so that the reference voltage Vref is increased for one bit. However, the processing time of the up/down counter 121 and/or the code-voltage converter circuit 124 causes a time delay Td before the reference voltage Vref increases for one bit.
On the other hand, when the clock signal CLK becomes HIGH at P1, the control signal PHS immediately becomes LOW (in level), so that the DC-DC converter 100 starts the switching of the switching transistor M1 by turning the switching transistor M1 ON. However, the reference voltage Vref at this time is still at a level before the clock signal CLK becomes HIGH at P1. Therefore, the reference voltage Vref is equal to the divided voltage Vfb of the output voltage Vo, and no change in the reference voltage Vref is reflected in the ON time of the switching transistor M1. Therefore, the output voltage Vo (voltage Vfb) does not change at P1 of the clock signal CLK.
The output voltage Vo starts changing at the inputting of P2 of the clock signal CLK. In this case also, however, the ON time of the switching transistor M1 is determined based on the reference voltage Vref that has changed at P1 of the clock signal CLK. Thus, the change of the output voltage Vo is always delayed by one clock cycle.
Also in the case of reducing the reference voltage Vref, there is a problem in that the change of the output voltage Vo is always delayed by one clock cycle as indicated by P5 and P6 of the clock signal CLK in FIG. 3.